From: | Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> |
---|---|
To: | Kevin Grittner <kgrittn(at)ymail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <stark(at)mit(dot)edu> |
Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com>, Jim Nasby <jim(at)nasby(dot)net>, Josh Berkus <josh(at)agliodbs(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: ANALYZE sampling is too good |
Date: | 2013-12-11 20:12:20 |
Message-ID: | 52A8C724.1050009@archidevsys.co.nz |
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Lists: | pgsql-hackers |
On 12/12/13 08:39, Gavin Flower wrote:
> On 12/12/13 08:31, Kevin Grittner wrote:
>> Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> wrote:
>>
>>> For example, assume 1000 rows of 200 bytes and 1000 rows of 20 bytes,
>>> using 400 byte pages. In the pathologically worst case, assuming
>>> maximum packing density and no page has both types: the large rows
>>> would
>>> occupy 500 pages and the smaller rows 50 pages. So if one selected 11
>>> pages at random, you get about 10 pages of large rows and about one for
>>> small rows!
>> With 10 * 2 = 20 large rows, and 1 * 20 = 20 small rows.
>>
>> --
>> Kevin Grittner
>> EDB: http://www.enterprisedb.com
>> The Enterprise PostgreSQL Company
> Sorry, I've simply come up with well argued nonsense!
>
> Kevin, you're dead right.
>
>
> Cheers,
> Gavin
>
>
I looked at:
http://www.postgresql.org/docs/current/interactive/storage-page-layout.html
this says that each row has an overhead, which suggests there should be
a bias towards small rows.
There must be a lot of things going on, that I'm simply not aware of,
that affect sampling bias...
Cheers,
Gavin
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